build: Put RPMs in architecture-specific subdirectories

Until now we have been putting all the RPMs we build in RPMS/x86_64.   Now that we also build noarch RPMs, we should have a more appropriate directory structure.

**N.B.:** After merging this change you will need to update `/etc/mock/xenserver.cfg` on your build machine and `/etc/yum/repos.d/xapi.repo` on any machines which are using this repository.